We only run a max of 300 per server. We have over 2000 users, but scattered throughout the US with remote Exchange boxes. If all the Exchange Boxes are next to the SQL database and BES Server you can max it out. But if you have remote boxes, then latency becomes a problem. If we push a server over 300 users, it can take up to an hour before a user will receive an e-mail on his blackberry when it came in on outlook earlier.